As I mentioned before, the bridge is (in a very general sense: you skip around a lot) designed from the top down. But it's built from the bottom up, and while the bridge-using public doesn't see it the foundation takes a fair bit of design. This is where geotechnical engineers get into the game: they're the ones that calculate the strength of the soil and rock in the area.
Any project, once it's decided about where the bridge/wall is going to be, starts with some soil borings. This essentially means take a drill rig and drilling into the earth some specified distance. Every so often (1.5-5 feet it typical, depending on whose ordering it and how deep you are) a soil sample is taken and analyzed. The main property being tested is the strength, which is inferred from multiple different measurements.
For a wall, a 30' (for those not "in the know", an apostrophe represents feet and a quotation is inches, so 30'-6" is 30 feet and 6 inches) boring is typical. For a bridge, 100' is not uncommon. Some footing types even require deeper borings, but that can get expensive quickly as equipment that goes that deep isn't cheap.
Pile driving is exactly what it sounds like. You take some amount of pile, stick it into the ground, and then continue to pound on it until it drives in deep enough to take the load. Essentially you just put a pneumatic hammer on top of it and turn it on. The hammer measures the resistance, and when you get enough you turn it off and start on the next pile. They don't start with 70'+ long piles, they're much shorter. As the pile starts to get close to running out of length, they pause the process and weld another length of pile onto the current one.
|Drilling a concrete shaft for the pier footing|
But we had to make a last minute change. There's a 60" sewer pipe directly beneath the pier foundation. Originally that was going to be grouted up (filled with a cementitious material: essentially turning it into a long, cylindrical rock) as it's old and a new one is coming to replace it. But just before the project was bid, some guys in the utilities world decided they wanted to keep the pipe in use for another couple of years before closing it off. We weren't sure the pipe could take the extra stress that would come from loading the soil on top of it (our shallow foundation) so we switched foundation types.
|The edge of the wall can be seen in the foreground. Most walls do not directly support the bridge, instead the force is transferred down behind the wall to the soils deep beneath it.|
|This specialized equipment was driven out from the Rockies, once the driller has done it's work, a rebar "cage", pre assembled steel bars, will be lowered into the hole and concrete poured in to fill it up.|
When you've built your foundation, you fill up all the excess area with sand and get started on the rest of the substructure. The sand has all sorts of fancy names and criteria but it's essentially just sand. Despite the parable, sand is actually a pretty good material to build on, but more importantly: it drains water. You take a lot of precautions to avoid having water just sit right next to your concrete footing. The concrete doesn't mind (as long as it doesn't freeze and thaw too much) but eventually the water penetrates and hits the rebar. This is bad. So you put quick-draining materials everywhere and then make sure there's a piece of perforated drain pipe somewhere nearby to take all that water away from your structure and get it out of there.